The brain and spinal cord are encased within the cranium and vertebral column inside a thin membrane known as the arachnoid. The volume of the intracranial space is on average about 1700 mL. The volume of the brain is approximately 1400 mL, the volume of the intracranial blood is approximately 150 ml; the remaining 150 mL is filled with cerebrospinal fluid. The cerebrospinal fluid circulates within the subarachnoid space. It is formed principally by the choroid plexuses, which secrete about 80% of the total volume. The sources of the remainder are the vasculature of the subependymal regions, and the pia mater. The total volume of the cerebrospinal fluid is renewed several times per day, so that about 500 mL are produced every 24 hours.
The cerebrospinal fluid is absorbed through the arachnoid villi, located principally over the superior surfaces of the cerebral hemispheres. Some villi also exist at the base of the brain and along the roots of the spinal nerves. The absorptive processes include bulk transport of large molecules and as well as diffusion across porous membranes of small molecules. (Adams et al., (1989) "Principles of Neurology," pp. 501-502.).
The principle on which this invention is based is that in some persons with adult-onset dementia of the Alzheimer's type there is dysfunction of the cerebrospinal fluid resorptive mechanism, leading to the retention in the cerebrospinal fluid of substances which result in the histologic lesions associated with adult-onset dementia of the Alzheimer's type, or which are neurotoxic, or both.
There are several examples of low-molecular weight proteins or peptides that are known to be present in elevated concentrations in the cerebrospinal fluid of persons suffering from adult-onset dementia of the Alzheimer's type. For example, elevated levels of beta A-4 amyloid have been found in the cerebrospinal fluid of patients with early-onset Alzheimer's disease. (Nakamura et al., (1994) "Amyloid beta protein levels in cerebrospinal fluid are elevated in early-onset Alzheimer's disease," Ann. Neurology 36: 903-911). Beta A-4 amyloid is known to self-aggregate into molecules of amyloid of the type that typify the core plaques found in the brain in persons suffering from adult-onset dementia of the Alzheimer's type. In fact, beta A-4 amyloid deposition in the brain is the only microscopic lesion specific for Alzheimer's disease. Furthermore, beta A-4 amyloid has been shown to be neurotoxic. (Bush et al., (1992) "Beta A-4 amyloid protein and its precursor in Alzheimer's disease," Pharmac. Tera. 56: 97-117). Beta A-4 amyloid is also a component of microscopic cerebrol lesions known as neurofibrillary tangles, characteristically found in adult-onset dementia of the Alzheimer's type.
Beta-2 microglobulin is another example of a low-molecular-weight protein whose concentration in the cerebrospinal fluid increases with age and reaches high levels in patients with adult-onset dementia of the Alzheimer's type. (Martinez et al., (1993) "Relationship of interleukin-1 beta and beta.sub.2 -microglobulin with neuropeptides in cerebrospinal fluid of patients with dementia of the Alzheimer type," J. Neuroimmunology 48: 235-240). Beta-2 microglobulin is associated with amyloid deposits in some tissues of patients on long-term renal hemodialysis. (Ono et al., (1994) "Formation of amyloid-like substance from beta-2-microglobulin in vitro. Role of serum amyloid P component: a preliminary study," Nephron 66: 404-407).
Another substance that accumulates in the cerebrospinal fluid in patients with adult-onset dementia of the Alzheimer's type is tau, a component of the neurofibrulary tangles found in involved brain tissue. Tau concentrations in cerebrospinal fluid are regularly increased in this syndrome with eight fold increases present in half of the patients. (Arai et al., (1995) "Tau in cerebrospinal fluid: a potential diagnostic marker," Ann. Neurology 38: 649-52.)
In addition, the prior art also describes devices used to remove cerebrospinal fluid from a patient. Matkovich U.S. Pat. No. 5,334,315 describes a method and device that can be used to remove a body fluid from a patient, to treat that fluid to remove an undesirable component, and to return the fluid to the patient. Matkovich's partial list of the kinds of deleterious or undesirable substances that can be removed from a fluid includes proteins, polypeptides, interleukins, immunoglobulins, proteases and interferon. The fluids from which these substances can be removed using the Matkovich apparatus include cerebrospinal fluid, blood, urine and saliva. Matkovich never suggests, however, that his method and apparatus could be used to treat patients suffering from adult-onset dementia of the Alzheimer's type.
Kirsch et al. U.S. Pat. No. 5,385,541 describes a cerebrospinal fluid shunt mechanism used to treat hydrocephalus by draining cerebrospinal fluid into the patient's abdomen, chest or vascular system. The system may include a one-way valve to prevent backflow. Kirsh does not describe the use of such a system to treat adult-onset dementia of the Alzheimer's type, however.
Ruzicka et al. U.S. Pat. No. 4,950,232 discloses another cerebrospinal fluid shunt system. As with the Kirsch et al. patent, Ruzicka does not suggest the use of his shunt system to treat adult-onset dementia of the Alzheimer's type.